If you mean the controls you have a few options. If you know your way around the cab of your chosen train you can manipulate the controls directly with the mouse- personally I find that quite hit and miss. You can use the keyboard and/or a game pad plus the mouse for quirky things that those don't cover like opening windows. There's also the Head Up Display (HUD) which, usefully, gives you a standard set of controls (or as standard as they can be given the differences between steam, Diesel and electric so there's some variation) and some route information such as speed and the current speed limit, distance to the next objective if it's close, gradients, etc. If you've got money to burn there's the Raildriver which is a customised control panel.

These aren't alternatives to each other- you can start using the mouse with the HUD and, as you get used to the keyboard controls, it becomes a display telling you how your controls are set. Use the keyboard (or game pad or Raildriver) to open the throttle for example and the throttle control in the HUD will will move to reflect that as will the lever you can see in the cab.